CHUKCHI SEA, OFF THE COAST OF NORTHERN ALASKA — On an all-but-invisible horizon, light gray mist meets dark gray water. Tiny, dry snowflakes and ice crystals swirl across the deck of the drillship Noble Discoverer, driven by a wind that whips through the warmest coveralls.

Just another day at the office, 400 miles above the Arctic Circle.

“It is a challenging environment,” conceded Shell Alaska Vice President Pete Slaiby, watching the waves that rocked the rig as it bored the first 1,500 feet of an exploratory well in the Chukchi Sea. “You have to plan every day, every operation.”

Six years and about $5 billion into its quest for Arctic oil, Shell still struggles to overcome the obstacles of this forbidding frontier, where the cold locks up machines and blankets of fog sometime keep planes out of the sky for days at a time.

Despite exhaustive planning and simulation of the work to come, problems started even before Shell's drilling units arrived. First, sea ice clung to Alaska's shores, preventing Shell's ships from cruising to their shallow-water drill sites. Then the Discoverer dragged its anchors and briefly floated out of control near Dutch Harbor, Alaska. Later, Shell confessed that it couldn't satisfy some terms of an air pollution permit governing the Discoverer. And the company's first-of-its-kind oil spill containment barge was damaged during certification tests.

Although oil companies punched nearly three-dozen wells in the Beaufort and Chukchi seas between 1982 and 1997, Shell's latest Arctic venture is the first bid to find crude underneath these waters in decades.

In its return to the Arctic, Shell navigated around such obstacles as low-hanging fog and floating ice while managing more than two dozen ships and the logistics of deploying, feeding and boarding hundreds of workers at a time.

Now, just days after Shell left its half-finished wells in the Chukchi and Beaufort seas for the winter, the company is preparing to make changes because of the lessons it learned this summer.

Slaiby thinks his company's logistical management was a success, in part because workers on the rigs never went without needed equipment.

Environmentalists say Shell's problem-plagued start proved the company isn't capable of safely searching for oil in the Arctic. They also say the government and public deserve more details about what went wrong with the containment system testing and the Discoverer in Dutch Harbor.